To send data from a contactless data storage medium to a read/write unit, it is known practice to vary the power drawn by the contactless data storage medium from the electromagnetic field generated by the read/write unit. The change in power drawn can be detected by the read/write unit and in this way data transmitted by means of appropriate modulation and demodulation can be detected. A currently applicable standard for such contactless data storage media or chip cards is ISO 14443 or ISO 15693. The known data storage media operate using resistive or capacitive loading modulation. For modulation purposes, the size of the load connected in parallel with the principal circuit component is thus increased, so that the power drawn by the contactless data storage medium power increases.
A problem with loading modulation is that the loading for modulation purposes also lowers the voltage which is available for supplying the principal circuit component. This can result in operating faults, for example as a result of an undervoltage detector circuit responding or else the clock detection circuit, which evaluates a clock signal transmitted using the electromagnetic field from the read/write unit, failing. Although it is possible to keep down the power level change, that is to say the level of load connection, a minimum modulation amplitude is required in order for the read/write unit to be able to detect the modulation. Overall, the loading modulation results in unwanted range limitation. Under these constraints, the load resistors are normally dimensioned with a compromise.
In addition, nonlinear modulation is also known, where the depth of modulation decreases before the supply voltage reaches the lower permissible voltage threshold. In the case of this form of a contactless data storage medium too, however, the modulation signal becomes weaker, which means that the likelihood of erroneous data transmission increases.
To get around these problems, it is known practice to provide offloading modulation instead of loading modulation. A contactless data storage medium operating on the basis of such a method is known from EP 1 042 731 B1, for example. The circuit shown in this document is provided with a shunt transistor which can be connected and disconnected. In this case, the power level change is dependent on the power dissipated in the shunt. In a normal operating state, when no data are intended to be sent, the shunt is connected and increases the current drawn by the contactless data storage medium. For modulation purposes, the shunt is disconnected, which results in a reduction in the drawn current, which in turn can be detected by the read/write unit. In the read/write unit, the contactless data storage medium's modulation signals are AC coupled, so that just the spectral amplitude of the modulation sidebands is taken into account. It therefore does not matter whether loading or offloading modulation takes place, which means that contactless data storage media operating on the basis of the principle of offloading modulation are also compatible with the existing read/write units. If the shunt has been disconnected in order to obtain the greatest possible range for the read mode, the shunt is not active and therefore also cannot be offloaded. It is therefore no longer possible to send data. In addition, the load connected in the normal operating state results in unwanted range limitation.